Traditionally, digitally controlled printing capability is accomplished by one of two technologies. The first technology, commonly referred to as “continuous stream” or “continuous” ink jet printing, uses a pressurized ink source which produces a continuous stream of ink droplets (typically containing a dye or a mixture of dyes). Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When print is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism.
The second technology, commonly referred to as “drop-on-demand” ink jet printing, provides ink droplets (typically including a dye or a mixture of dyes) for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.
Conventional “drop-on-demand” ink jet printers utilize a pressurization actuator to produce the ink jet droplet at orifices of a print head. Typically, one of two types of actuators are used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink droplet to be expelled. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
Conventional ink jet printers are disadvantaged in several ways. For example, in order to achieve very high quality images having resolutions approaching 900 dots per inch while maintaining acceptable printing speeds, large numbers of discharge devices located on a printhead need to be frequently actuated. The frequency of actuation limits the viscosity range of the ink used in these printers. Typically, the viscosity of the ink is lowered by adding solvents such as water, etc. The presence of solvents can cause an increase in ink bleeding during drying which reduces image sharpness negatively affecting image resolution and other image quality metrics. Additionally, the presence of solvents results in slower ink drying times after the ink has been deposited on the receiver which decreases overall productivity.
Conventional ink jet printers are also disadvantaged in that the discharge devices of the printheads can become partially blocked and/or completely blocked with ink. In order to reduce this problem, solvents, such as glycol, glycerol, etc., are added to the ink formulation, which can adversely affect image quality due to an increase in ink bleeding during the time the ink is drying.
In conventional ink jet printing, when an overcoat is desired, the ink is allowed to dry prior to applying the overcoat. Again, the presence of solvents results in slower ink drying times after the ink has been deposited on the receiver. Therefore, overall printing system productivity is reduced due to the waiting period associated with increased drying times.
When a precoat, typically containing solvents, is desired, the precoat is usually allowed to dry prior to the commencing the printing process. Allowing the precoat to dry reduces the likelihood of ink bleeding when the ink is applied to the receiver. The time associated with drying reduces the overall printing system productivity.
Other technologies that deposit a dye onto a receiver using gaseous propellants are known. For example, Peeters et al., in U.S. Pat. No. 6,116,718, issued Sep. 12, 2000, discloses a print head for use in a marking apparatus in which a propellant gas is passed through a channel, the marking material is introduced controllably into the propellant stream to form a ballistic aerosol for propelling non-colloidal, solid or semi-solid particulate or a liquid, toward a receiver with sufficient kinetic energy to fuse the marking material to the receiver. There is a problem with this technology in that the marking material and propellant stream are two different entities and the propellant is used to impart kinetic energy to the marking material. When the marking material is added into the propellant stream in the channel, a non-colloidal ballistic aerosol is formed prior to exiting the print head. This non-colloidal ballistic aerosol, which is a combination of the marking material and the propellant, is not thermodynamically stable/metastable. As such, the marking material is prone to settling in the propellant stream which, in turn, can cause marking material agglomeration, leading to nozzle obstruction and poor control over marking material deposition.
Technologies that use supercritical fluid solvents to create thin films are also known. For example, R. D. Smith in U.S. Pat. No. 4,734,227, issued Mar. 29, 1988, discloses a method of depositing solid films or creating fine powders through the dissolution of a solid material into a supercritical fluid solution and then rapidly expanding the solution to create particles of the marking material in the form of fine powders or long thin fibers, which may be used to make films. There is a problem with this method in that the free-jet expansion of the supercritical fluid solution results in a non-collimated/defocused spray that cannot be used to create high resolution patterns on a receiver. Further, defocusing leads to losses of the marking material.
As such, there is a need for a technology that permits high speed, accurate, and precise delivery of solvent free marking materials to a receiver to create high resolution images. There is also a need for a technology that permits high speed, accurate, and precise imaging on a receiver having reduced material agglomeration characteristics.